Catalysts for cleanup of internal combustion engine exhaust gases conventionally consist of an optionally porous honeycomb of ceramic tubes. Exhaust gas flows axially through the exhaust tubes with reactions taking place at active sites on the tube walls. These reactions convert pollutant compounds in the exhaust gas into environmentally acceptable compounds. Diffusion of gaseous pollutant chemical species from the bulk, axial exhaust gas flow to the active surface sites located on the walls is needed to achieve high conversion—either oxidation or reduction of pollutant species.
Particulate filters used for diesel exhaust particulate matter reduction may consist of a honeycomb of tubes having porous walls. The exhaust gas is forced to flow through the porous walls, such as by blocking alternate ends of adjacent tubes, and the particulate matter is captured in the pores where the particulate matter may be catalytically oxidized.
Current conventional diesel engines create exhaust flows having a NOx content of anywhere from 0 ppm NOx at no load to 2000-5000 ppm NOx at full load. In the context of the present invention, NOx is used to mean any and all oxide of nitrogen, including but not limited to NO, NO2, and N2O. At present, there is no exhaust after-treatment technique capable of causing diesel engines to meet the standards for emissions proposed by the U.S. EPA to take effect in 2010. NOx reduction catalysts (“lean NOx catalysts”) using fuel injection ahead of the catalyst do not offer the 90% or greater reduction in NOx levels required by the EPA 2010 standards. Selective catalytic reduction technology requires the injection of some additional reductant such as urea, which is not favored and further requires careful control to avoid ammonia slip. NOx adsorber technology shows promise but does not provide the reductions required over a broad operating range.
A problem with diesel exhaust NOx reduction is that excess oxygen is typically present in the exhaust, while the exhaust temperature and exhaust species of interest can vary significantly in concentration and amount. Any scheme which provides for the reduction of NOx must either a) avoid reactions between this excess oxygen and nitrogen or the added reductant, or b) provide adequate reducing agent that the exhaust mixture composition approaches the composition of exhaust from combustion at a stoichiometric air-fuel ratio.
NOx absorber or trap technology utilizes rare earth absorption sites that serve to trap NOx molecules under lean conditions, i.e., with an excess of oxygen in the fuel/air mixture. The adsorption sites are periodically regenerated by injection of sufficient reductant such that the overall amount of oxygen in the exhaust is reduced to a negligible level (reductant rich). The reduction reaction is then promoted at or near the adsorption sites, producing elemental nitrogen and oxidized products such as water and/or carbon dioxide. The regenerated sites are then available for further NOx absorption. The drawbacks of this approach are the periodic requirement of having a reductant rich exhaust and the associated inherent inefficiency in transporting the reductant to the active sites. These drawbacks lead to significant fuel efficiency penalties and problems in controlling the exhaust composition.
The above-described problems exist not only in diesel engines, but also in other type of engines as well, regardless of whether the engine is compression ignited or spark ignited, or whether the fuel is diesel, gasoline natural gas, alcohols, hydrogen or some alternative liquid or gaseous fuel. What is needed is an improved method and apparatus for reducing the level of NOx emissions in internal combustion engine exhaust gases. The method and apparatus should minimize losses in fuel efficiency and/or minimize the amount of added reductant or reactant. The method should also allow for low cost of operation. The method and apparatus should further be adaptable to conventional internal combustion engine designs.